Validate HTML code for syntax errors and compliance.
The HTML Validator checks your markup for the errors that break pages: tags that are never closed or closed in the wrong order, stray closing tags, duplicate ids and attributes, unclosed quotes and comments, and misuse of void elements like <br> and <img>. It understands HTML's optional end tags, so valid shortcuts like unclosed <li> or <p> are not flagged, and it also warns about missing alt text, obsolete tags and a missing doctype, title or lang attribute. Every issue is listed with its line and column, checked locally in your browser.
Made for developers. Typical uses:
Syntax and format checks find broken markup, bad JSON and mistyped numbers, but not meaning, business rules, email deliverability or whether a URL is alive.
What minifying removes, how much it saves next to gzip and Brotli, and how to test minified CSS, JavaScript and HTML before you deploy it.
Explore more in Validators, read the guides and articles, or see how we test our tools.
Validate Base64 encoded strings for correctness.
Best for confirming an API response's Base64-encoded field wasn't truncated during a copy-paste
Validate credit card numbers using Luhn algorithm.
Best for validating credit card numbers in e-commerce
Validate CSS code for syntax errors and compliance with W3C standards.
Best for catching a missing semicolon or unclosed brace that's silently breaking a stylesheet
Validate date strings for correct format, leap years, and logical validity.
Best for validating a user-submitted birthdate in a signup form before saving it